Power amplifiers are widely used, especially in radio communication applications, for example, but also in many other applications. The specifications for such power amplifiers often require the amplifier to be capable of producing at least a minimum output power level even in adverse circumstances of high ambient temperature and low battery charge.
Typically, such power amplifiers include a feedback loop for the power control of the amplifier that includes a detector responsive to the power of the amplifier output signal; the feedback loop compares the detector response with a received power target, in the form of a first power target signal, and controls the output power to a corresponding level.
Problems arise in some situations when the power amplifier output reaches a value at which the amplifier is saturated, that is to say that the amplifier is operating at maximum power and cannot increase further its output power in response to the power target. A particular problem of this kind arises in the case of certain cellular telephone systems, such as the Global System for Mobile Communications (“GSM”) or the third generation telephone systems (“3GPP”), for example that users time division multiple access (“TDMA”) communication protocols. Such protocols include time slots allocated to the user terminals; during the allocated time slot, the power amplifier of the user terminal is required to ramp up to the power indicated by the power target, send the desired signal, and ramp down to a much lower power level so as not to interfere with users sharing the same frequency in other time slots. The standard specifications for the protocols include a time mask and a spectral frequency mask which the terminals must meet. These specifications require a smooth ramp-up of the power amplifier output under strict time constraints.
Not only does saturation of the power amplifier risk impacting the efficiency and quality of operation of the power amplifier, but also the saturated power amplifier may take an excessive time to ramp down its power.
U.S. Pat. No. 5,278,994 provides a response to this problem. The amplifier controller detects saturation of the amplifier, by responding to a parameter such as lack of reaction of the amplifier output to the power target, in the form of a feedback error signal that does not diminish, and adjusts the output signal power to a level at which the power amplifier is substantially not saturated, that is to say that the power amplifier operates at or close to its maximum power without being saturated.
If all components of the feedback loop had perfect characteristics, with no manufacturing tolerances and no variability in operation, this system would be very satisfactory. However, in practice, the production test equipment itself has tolerances which cause variability of the power detector response and the power detector response varies in addition with temperature and frequency. The order of magnitude of the production tolerances and of the variability of response with temperature and frequency (typically of the order of 0.5 dB in each case) are significant. In particular, in order to meet the minimum output power requirements of the specifications referred to above, the power amplifier is oversized typically, that is to say has a maximum output power greater than that required by the specification, in order to ensure that even with unfavourable variance of the detector response, the power amplifier will still be capable of delivering the specification minimum output power.
This oversizing of the power amplifier penalises the talktime, that is to say the accumulative length of time during which the user terminal can operate transmitting and receiving content before the battery needs recharging, and represents a cost penalty also.
There is a requirement for a power amplifier system which reduces or avoids such oversizing.